Injection of an electrically heated stream into a steam cracked product

ABSTRACT

A STEAM CRACKING FEED MIXTURE CONTAINING STEAM AND HYDROCARBON IS CONVENTIONALLY CRACKED IN THE TUBES OF A STEAM CRACKING FURNACE. THE CONVERSION OF HYDROCARBON TO CRACKED PRODUCT CAN BE INCREASED BY INJECTING A HIGH TEMPERATURE GASEOUS STREAM SUCH AS CAN BE PRODUCED BY ELECTRICAL HEATING, INTO THE MIXTURE IN THE CRACKING FURNACE OR INTO THE MIXTURE AS IT EMERGES FROM THE CRACKING FURNACE, THUS SUBJECTING THE FURNACE EFFLUENT TO SHORT RESIDENCE TIME, HIGH TEMPERATURE CRACKING CONDITIONS. IN ONE METHOD, METHANE IS ELECTRICALLY HEATED AND CRACKED TO A MIXTURE COMPRISING ACETYLENE AND HYDROGEN, WHICH IS THEN INJECTED INTO THE PRODUCT FROM THE STEAM CRACKER, THEREBY CAUSING ADDITIONAL CRACKING OF THE STEAM CRACKER FEED AND QUENCHING OF THE ELECTRICALLY CRACKED PRODUCT TO PRESERVE THE ACETYLENE CONTENT THEREOF. THE LECTRICALLY HEATED STREAM MAY, HOWEVER, CONTAIN HYDROGEN, STEAM, HYDROCARBONS, E.G. ACETYLENE, OR MIXTURES THEREOF AS THE COMPONENTS OF GREATEST CONCENTRATION.

Feb. 8, 1972 p, c EI'AL 3,641,183

INJESTION OF AN ELECTRICALLY HEATED STREAM INTO A STEAM CRACKED PRODUCT Filed-July 9, 1968 By Q Aflornay United States Patent U.S. Cl. 260-683 15 Claims ABSTRACT OF THE DISCLOSURE A steam cracking feed mixture containing steam and hydrocarbon is conventionally cracked in the tubes of a steam cracking furnace. The conversion of hydrocarbon to cracked product can be increased by injecting a high temperature gaseous stream such as can be produced by electrical heating, into the mixture in the cracking furnace or into the mixture as it emerges from the cracking furnace, thus subjecting the furnace efiluent to short residence time, high temperature cracking conditions. In one method, methane is electrically heated and cracked to a mixture comprising acetylene and hydrogen, which is then injected into the product from the steam cracker, thereby causing additional cracking of the steam cracker feed and quenching of the electrically cracked product to preserve the acetylene content thereof. The electrically heated stream may, however, contain hydrogen, steam, hydrocarbons, e.g. acetylene, or mixtures thereof as the components of greatest concentration.

FIELD OF THE INVENTION This invention relates to an imporved steam cracking process. More specifically, this invention relates to a steam cracking process wherein the conversion of hydrocarbon feed to cracked product is increased by injecting into the steam cracking mixture, while still in the furnace but preferably as it leaves the furnace, an electrically heated gaseous stream at a temperature higher than that of the steam cracking mixture. The electrically heated gaseous stream may comprise, as the component, in greatest concentration, steam, hydrogen, electrically cracked hydrocarbons or mixtures of the foregoing. In a preferred embodiment, methane or other low value hydrocarbon material produced in conventional steam cracking operations is separated from the steam cracking product, electrically heated and cracked in high conversions to acetylene and hydrogen, and the hot acetylene-hydrogen mixture is injected into the steam cracking product to further crack the hydrocarbon feed and quench the electrically cracked product to preserve the acetylene. In this manner, rather low value products can be recycled to extinction and utilized to produce more valuable products.

PRIOR ART Conventional steam cracking processes are generally well known to the art and have been widely used in the preparation of valuable unsaturated materials, eg ethylene, propylene, butadiene, particularly ethylene. Nevertheless, the yield of cracked products such as ethylene and the level of conversion of hydrocarbon feed to the cracking furnace is greatly limited by metallurgy, coking tendency, and heat transfer rates. Thus, steam cracking is effected by passing a mixture of steam and a hydrocarbon feed through metal tubes arranged in a cracking furnace. The tubes are then exposed to convective and radiant heat which is transferred through the tube walls and to the reactants Within the tubes, thereby causing the hydrocarbon feed to crack. Of course, the higher the temperature to which the hydrocarbon feed can be exposed 3,641,183 Patented Feb. 8, 1972 the greater the extent of the cracking process, i.e., greater conversion. (Conversion is defined as weight percent of feed converted to C and lighter material.) However, available metallurgical techniques have not yetdevis ed tube metal materials that can continuously stand up to metal temperatures in excess of about 2000 F. Further, inhibiting the cracking of the feed is the tendency of the feed, when cracked, to deposit coke on the interior of the tube walls. Now, since coke is a thermal insulator, i.e. has poor heat transfer qualities, less heat is transmitted to the feed, thereby lowering the temperature level and reducing the amount of cracking that can be effected. Obviously, raising furnace temperatures is not satisfactory in view of the maximum operating limit of the tube metal already mentioned. Because of the foregoing factors, then, conversions in conventional steam cracking processes are limited to about 3050% on feedstocks in the gas oil boiling range, and to 5070% on naphtha feeds; ethylene yields, depending on feedstock, are limited to about 20- 30%. However, by utilizing the process of this invention, which allows a simple method for boosting the temperature of the steam cracking reaction mixture, markedly increased overall conversions and increased ethylene yields can be obtained. For example, conversions can be increased by 50% or more to about +C weight percent on feed, preferably even with heavy feedstocks, and C -unsaturate, e.g. ethylene, acetylene, yields can similarly be increased by a factor of 1.2-2 up to about 35-45% or even higher.

SUMMARY OF THE INVENTION In accordance with the invention, therefore, an improved cracking process is provided which comprises passing steam in admixture with a suitable hydrocarbon feed through metal tubes arranged in a cracking furnace, exposing the tubes to sufficient convective and radiant heat, such that the heat transferred to the reaction mixture in the tubes is sufficient to crack the hydrocarbon passing therethrough, and injecting into the cracked product, either while it is still within the radiant section of the cracking furnace or after emerging from the cracking furnace, a gaseous electrically heated stream having a component preferably of greatest concentration selected from the group consisting of inert gases, hydrogen, steam, cracked hydrocarbons, e.g. acetylene, and mixtures thereof, thereby subjecting the cracked product to short residence time, high temperature cracking conditions, the electrically heated stream being at a higher temperature than the steam cracked product mixture and causing additional cracking of the hydrocarbon feed to the steam cracking process. In a preferred embodiment of this invention, any excess hydrocarbon product from the steam cracking operation, preferably methane, is separated downstream from the steam cracker, passed through an electric arc which cracks the methane to a gaseous stream rich in hydrogen and acetylene, the stream being quenched by injecting it into the steam cracker efiluent, thereby providing sufiicient additional heat to cause further cracking of unconverted hydrocarbon feed.

DRAWING DESCRIPTION steam cracking furnaces contain two sections, i.e. a convection section wherein the feed is vaporized, if not already in the vapor state, and heated to about 1000-1100 F. but below that temperature at which cracking starts, and, a radiant section where the vaporized feed is quickly heated to temperatures ranging from 1200 to about 1800 F. and cracked. The product of the cracking furnace comprising steam, cracked product, and unconverted feed, in line 13 is then mixed as soon as possible after leaving the furnace, e.g., before any significant temperature loss, with a gaseous electrically heated stream which is at a temperature of about 2500-4000 F. in line 14. The additional heat supplied by the stream in line 14 is sufiicient to cause additional cracking of the unconverted hydrocarbon feed in post-reactor 16, after which the total product in line 17 is introduced into heat exchanger or quench line 18 wherein a quench oil from line 19 can be added to inhibit any further reaction. The quenched product at about 300-700 F. in line 20 is then transferred to a fractionator 21 wherein the various products are separated, for example, hydrogen in line 27, acetylene in line 23, ethylene and ethane in line 24, and in line 25. Methane, however, in this embodiment, is separated in line 22 and sent to pre-heater 26 wherein the temperature is raised to about 500 to 1500" F. and thence to electric arc heater 27 wherein the methane is cracked, in large part, to hydrogen and acetylene in line 14. Some of the hydrogen in line 27 may be recycled (not shown) to the arc to suppress coke formation when cracking a hydrocarbon.

PREFERRED EMBODIMENTS The process described herein may take on various forms or embodiments, some of which are preferred over others and which are described hereinbelow.

The feed to the steam cracking operation can contain any suitable hydrocarbon, i.e. one that will crack to yield desirable unsaturated products. Generally, however, the steam cracking process is most applicable to the cracking of feeds consisting essentially of cyclic or acyclic hydrocarbons, preferably saturated hydrocarbons. Thus, hydrocarbon materials which can be utilized as feed stocks include such cyclic hydrocarbons as cyclopropane, cyclobutane, cyclopentane, cyclohexane, cyclooctane, cyclododecane, etc., and mixtures thereof. Acyclic hydrocarbon feed can include any alkane, namely aliphatic hydrocarbons of the methane series or mixtures of such alkanes with cycloalkanes. Preferred feeds are those saturated hydrocarbons containing about 2 to about 24 carbon atoms, most preferably C -C alkanes and mixtures of such alkanes, eg. ethane, propane, butane, isobutane, hexanes, octanes, etc., n-hexadecane, eicosane and naphthas boiling in the range of 90430 F., gas oils of 450 800 F. boiling range, and kerosenes of 430-55 F. boiling range can also be effectively cracked in the process. The feed is generally diluted with steam to increase cracking efficiency and the feed mixture is usually comprised of about 20-80 rnol percent steam, preferably 20 60 mol percent, more preferably 3 0-60 mol percent steam.

It should be noted that the type of feed conventionally fed to a steam cracking furnace is limited by the tendency to deposit coke on the tube walls while it is being cracked to the conversion level required to meet a certain product demand. Thus, highly unsaturated and cyclic feedstocks tend to be heavier cokers, and are more refractory to undergo desirable cracking reactions than paraffins and naphthenes of comparable molecular weight. Similarly, higher boiling feedstocks will deposit more coke than lighter feedstocks when subjected to the same cracking severity. Since the present invention permits the most severe portion of the cracking reaction (both from a conversion level and a temperature level point of view) to be carried out in a zone where coking on the walls is not detrimental to heat transfer, etc., feedstocks pre viously considered undesirable can now be fed to a steam cracker. These stocks, such as olefins, highly aromatic fractions and heavy feeds (boiling above 800 F.) can be cracked to low severity (10-40%) in the steam cracker, but to any desired conversion level in the subsequent post-reactor.

As previously mentioned the feed, hydrocarbon and steam, is heated in a convection zone to about l000- 1100 F. but below cracking temperatures. The heated feed is then passed to a radiant zone where the steam cracking reaction occurs. Here, the tubes are heated to about 1800 =F.-2000 F. The residence time in the cracking zone ranges from about 0.1 to 10 seconds, preferably 0.1 to 1.0 second, the higher the temperature the lower the residence time to obtain a given conversion level. Pressures within the tubes are not critical and can range from about 0-50 p.s.i.g., but higher pressures, e.g. up to about p.s.i.g. can be tolerated. In order to maximize the yield of unsaturates, low hydrocarbon partial pressures are desirable. Steam acts as an effective diluent for this purpose.

Now, in order to increase the overall conversion of cracked feed and particularly the ethylene yield, a hot gaseous stream resulting from electrically heating a suitable material or materials is injected into the steam cracking product mixture either within the radiant section or as the efiluent emerges from the radiant section. Preferably, however, the gaseous stream produced by electrical heating is injected into the steam cracker efiiuent, i.e. outside the radiant zone. In this manner, the heat of cracking can be supplied in the furnace at a more favorable temterature level and maximum cracking in the furnace is obtained compared to injecting hot material at at intermediate point in the radiant coil. Generally, the gaseous stream from electrical heating is at a temperature ranging from about 20004000 F., preferably 23004000 F., more preferably 26004000 F. The hot electrically heated stream is mixed in sufficient quantity with the steam cracked product to raise the temperature of the mixed eflluent streams to below about 2500 F., preferably 1700- 2200 F., more preferably about l7502000 F. when the arc effluent contains acetylene. At these temperatures unconverted (or uncracked) hydrocarbon feed emerging from the radiant section is cracked and overall conversion of hydrocarbon feed is increased. As mentioned, the temperature of the mixed stream should not exceed about 2500 F., since above such temperatures the water-gas reaction leading to CO formation begins to become excessive. The temperature boost supplied by the electrically heated stream is generally maintained for a period of about 1-10 milliseconds before quenching to stop all further reaction.

The electrically heated hot gaseous stream utilized to boost the temperature of the hydrocarbon cracking feed can be obtained by using any suitable electric heating means, i.e., resistance elements, an electric are or plasmajet. Preferably, the electric arc is produced by what is generally known as a transpiration electrode. Briefly, a transpiration electrode consists of a porous anode through which the reactant is allowed to flow into the arc (see US. Pat. 3,209,193). Other arc heating devices which have been developed are described in the literature and can be advantageously employed in the present invention. They include the Knapsack-Griesheim arc (see US. Pat. 2,916,534), the Union Carbide arc (U.S. Pat. 3,051,639) where methane is cracked by means of electrically superheated hydrogen; also the Du Pont arc (Chemical Week, Jan. 18, 1964, pp. 64-65) and the Huels arc (Chemical Week, May 18, 1957, pp. 112-116), both of which have been operated commercially over extended periods of time.

The stream which is electrically heated can comprise a variety of ditferent materials so long as the effluent from the arc chamber does not contain CO or CO in quantities sufficient to contaminate the steam cracking product. For example, steam, or inert gases, e.g. nitrogen,

argon, etc. can be electrically heated and utilized to boost the temperature of the steam cracked product. However, these are not preferred materials due to the extra condensation and compression loads, respectively, placed on downstream separation facilities, and the contamination of the hydrogen product with argon or nitrogen, etc. A preferred material that can be electrically heated is hydrogen. Still more preferred materials are hydrocarbons which when subject to electric arc temperatures, e.g. above about 22002500 F., crack to mixtures rich in acetylene and hydrogen. The use of such materials is highly advantageous since acetylene is a valuable material which is also produced in steam cracking operations. However, the relatively low yield of acetylene from steam cracking, e.g. 0.51.0%, makes it impractical to recover it and acetylene is usually hydrogenated. By this process, however, added to the acetylene in the steam cracked product ,is the acetylene content of the electrically heated (cracked) hydrocarbons and when the temperature of the steam cracked product is raised after injection of the hot stream from electrical heating, additional acetylene is produced, e.g. 3-5% and, therefore, the acetylene is now present in suflicient quantity for separation and recovery. It should be noted that while acetylene recovery from the large quantity of cracked gases is an expensive process, its cost is compensated for somewhat by the fact that thorough removal and recovery of acetylene from the cracked gas obviates the need for the conventional acetylene hydrogenation facilities.

The hydrocarbon utilized in the electric are cracking can be any hydrocarbon, e.g. aliphatic, cyclic, aromatic, etc., ranging from C to about C preferably, C to C Most preferably, the hydrocarbon is methane or a stream which is primarily methane, i.e. natural gas, liquefied petroleum gas, methane-hydrogen streams, and the like. Nevertheless, it is most preferable to utilize any excess hydrocarbon produced in steam cracking by separating and recycling that product to the electric arc. Thus, such products as unconverted feed, steam cracked naphthas, C -C olefins and diolefins, and the like, which are in excess supply at any given time, i.e., of low value at that time, can be utilized as feed to the electric arc. It is noted that methane is also a product of steam cracking and, therefore, in the most preferred case, methane is sep arated and utilized as feed to the electric arc.

Since the high temperatures utilized in electric are cracking of hydrocarbons are conducive to carbon formation, it is generally desirable to employ hydrogen in the feed to the arc to inhibit carbon formation. Generally, the tendency towards carbon formation increases as the molecular weight of the feed to the electric arc increases. Usually, at least about 20% molecular hydrogen in the feed to the arc is sufiicent to inhibit carbon formation, and preferably about 50 to 200% hydrogen is present in all hydrocarbon feeds to the electric are so as to direct the cracking to acetylene rather than coke. In the case of the transpiration electrode, hydrogen content of the feed stream as low as 1-5 mol. percent can be tolerated with feeds which are normally gaseous (C -C and which do not reach their thermal cracking temperature (1000- 1400 F.) until they have passed the porous anode (vide supra). However, even with the transpiration are, it may be advantageous to have a 2075% concentration of hydrogen in the arc feed.

When an acetylene rich are eflluent is produced it is necessary to quench the eflluent rather quickly, e.g. about 1 millisecond, or at a rate of F./sec. (see Kamptner and Krause, HTP: After Five Years, Hydrocarbon Processing, vol. 45, No. 4, April 1966, p. 187) to below about 2000 F., preferably below about 1800 F., to stop further reaction and preserve the acetylene yield.

Having now described the inventive process, it will be further illustrated by reference to the following examples. However, no limitations are to be implied from these examples since variations and modifications thereof will be obvious to those skilled in the art.

EXAMPLE 1 A naphtha feedstock boiling in the C 350 F. range is cracked in a tubular furnace in the presence of steam. With a total residence time of 0.3 second and a 1550 F. coil outlet temperature, conversion of the feed to C and higher material is 63.7 wt. percent. The yield of ethylene is 30.4 Wt. percent, of acetylene 0.6 Wt. percent. The effluent from this cracker is then subjected to additional cracking by admixture with a hot, inert gas (argon), so that the temperature of the mixture is 19002000 F. for 1-l.5 l0- seconds before quenching to below 500 F. The conversion of the feed mixture to C and lighter material is increased to 75.6 wt. percent and the yields of ethylene and acetylene are raised to 38.5 and 3.5 wt. percent respectively.

EXAMPLE 2 A virgin gas oil feed stock boiling above 430 F. is cracked in a tubular furnace in the presence of steam. Coil outlet temperature of 1500 F. results in a C and lighter material production of 46.0 wt. percent on feed, including methane 9.0, ethylene 17.0 and acetylene 0.3 Wt. percent on feed. At the same time, a mixture consisting of mol percent methane, 5 mol percent hydrogen is passed through a high temperature electric arc which results in the formation of a 3000 F. gas product comprising 72.8 mol percent hydrogen, 2.6 mol percent methane, 3.9 mol percent ethylene and 18.3 mol percent acetylene. The efiluents from the tubular cracker and the electric arc chamber are rapidly mixed. The mixture reaches a temperature of 1750 F.1800 F. and is held for 23 l0 seconds before quenching to a temperature below 800 F. The yield of light material derivable from the gas oil feed stock is increased to the following values: Total C and lighter 65.1 wt. percent; methane 15.4, ethylene 29.9, acetylene 2.1 wt. percent.

EXAMPLE 3 A gas oil is cracked in a tubular furnace in accordance with Example 2 and the effluent mixed with electric arc eflluent to obtain a high severity, cracking temperature of 1750-1850" F. The effluent from this cracking zone is quecnhed and sepaarted into its components. Hydrogen is taken off as a 95% purity product, and 96% of the methane produced is recycled to the electric arc crack ing zone in admixture With a small amount of hydrogen. Overall yields of product from 1000' lbs. of feed include: H 1960 SCF; acetylene 107 lbs.; ethylene 350 lbs.

What is claimed is:

1. In a process for thermally cracking a hydrocarbon feedstock Which comprises passing a reaction mixture containing said feedstock and steam to a multiplicity of tubes arranged in a cracking furnace, subjecting the tubes to radiant heat sufficient to raise the temperature of the reactants in the tubes above the cracking temperature of the hydrocarbons, cracking the feed hydrocarbon, and recovering an effluent containing steam, cracked hydrocarbons, and unconverted feed hydrocarbon, the improvement which comprises injecting into the reaction mixture, after the cracking of the feed hydrocarbons has been initiated, an electrically heated gaseous stream which com prises a material selected from the group consisting of hydrogen, inert gases, steam, cracked hydrocarbons, and mixtures thereof, the gaseous stream being at a temperature ranging from about 2000-4000 F. in order to effect cracking of the unconverted feed hydrocarbon.

2. The process of claim 1 wherein the gaseous stream is injected into the reaction mixture while the reaction mixture is in the radiant section of the cracking furnace.

3. The process of claim 1 wherein the gaseous stream is injected into the reaction mixture after the reaction mixture leaves the cracking furnace.

4. The process of claim 3 wherein the gaseous stream injected into the reaction mixture comprises inert gases.

5. The process of claim 3 wherein the gaseous stream injected into the reaction mixture comprises hydrogen.

6. The process of claim 3 wherein the gaseous stream injected into the reaction mixture comprises steam.

7. The process of claim 3 wherein the gaseous stream injected into the reaction mixture comprises cracked hydrocarbons.

8. The process of claim 7 wherein the cracked hydrocarbons are predominantly acetylene.

9. The process of claim 8 wherein the gaseous stream also contains hydrogen.

10. In a process for thermally cracking a hydrocarbon feedstock which comprises passing a reaction mixture containing said hydrocarbon feedstock and steam through a multiplicity of tubes arranged in a cracking furnace, subjecting the tubes to radiant heat sufficient to raise the temperature of the tubes to a range of from about 1800 F. to about 2000 F., cracking the feed hydrocarbon in said tubes for a period of time in the range of from about 0.1 to about 1.0 second, and recovering an eflluent containing steam, cracked hydrocarbons, and unconverted feed hydrocarbon, the improvement which comprises separating a portion of the cracked hydrocarbons, passing the separated hydrocarbons to an electrical heating means, heating the separated hydrocarbons to a temperature ranging from about 2000-4000 F., thereby cracking the separated hydrocarbons, and injecting the electrically heated, cracked hydrocarbons into the eflluent from the cracking furnace thereby causing additional cracking of the unconverted feed hydrocarbon.

11. The process of claim 10 wherein the electrically heated, cracked hydrocarbons contain, in greatest concentration, acetylene.

12. The process of claim 10 wherein the separated hydrocarbons are mixed with hydrogen prior to electrical heating.

13. The process of claim 10 wherein the separated hydrocarbons are electrically heated to temperatures ranging from about 2300-4000 F.

14. The process of claim 10 wherein the effiuent from the steam cracker is at a temperature ranging from about 1200 F.-1800 F.

15. The process of claim 10 wherein the portion of hydrocarbons separated from the eflluent of the cracking furnace is primarily methane.

References Cited UNITED STATES PATENTS 1,823,503 9/1931 Mittasch et al 260-679 2,185,041 12/1939 Stapleton 196-58 2,022,810 12/1939 Mekler 196-48 2,642,466 6/1953 Garner et al 208-72 2,158,812 5/1939 Alther 208-72 2,126,204 8/ 1938 Morrell 196-60 1,585,573 5/1926 Thomas 204-168 2,925,329 2/1960 Yost 23-281 2,147,551 2/1939 Saives 23-277 2,028,795 1/1936 McKee 208-72 3,051,639 8/1962 Anderson et a1. 204-171 2,245,819 6/1941 Porter 208-130 2,859,258 11/1958 Fischer et a1 260-683 2,220,795 11/ 1940 Smith 208-72 FOREIGN PATENTS 783,567 9/1957 Great Britain 208-130 DELBERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner U .8. Cl. X.R. 

